Matrix assembly for aligning electron multiplier components

ABSTRACT

An apparatus for precisely locating the components at the input and output of an electron multiplier which includes mounting plates having precisely located holes for securing a plurality of arms in a predetermined arrangement. Mounted on the arms of the compoents which are fixed in their proper position when the apparatus is assembled.

United States Patent [72] lnventor Willaim H. Shriner Blanchester, Ohio[21 Appl. No. 867,997 [22] Filed Oct. 21, 1969 [45] Patented Nov. 23,1971 [73] Assignee The Bendix Corporation [54] MATRIX ASSEMBLY FORALIGNING ELECTRON MULTIPLIER COMPONENTS 1 Claim, 4 Drawing Figs.

[52] U.S.Cl 313/103, 250/41.9 R, 313/243 [51] Int. Cl ..H0lj 43/16, H01j39/34, BOld 59/44 [50] Field of Search 313/103- [56] References CitedUNITED STATES PATENTS 2,615,135 10/1952 Glenn,.lr. 250/41.9

Primary Examiner-Herman Karl Saalbach Assistant ExaminerMarvin NussbaumAttorneys-Raymond .l. Eifler and Plante, Arens, Hartz, Smith andThompson ABSTRACT: An apparatus for precisely locating the components atthe input and output of an electron multiplier which includes mountingplates having precisely located holes for securing a plurality of armsin a predetermined arrangement. Mounted on the arms of the compoentswhich are fixed in their proper position when the apparatus isassembled.

PATENTEUuuv 23 I97! SHEET 1 [IF 2 N WEDGE WILLIAM H. SHRINER INVENTOR.

BY ATTORNEY PAIENTEDuuv 23 I971 SHEET 2 OF 2 FlGURE 3 FlGURE 4 WILLIAMH. SHRINER INVENTOR.

ATTORNEY MATRIX ASSEMBLY F OR ALIGNING ELECTRON MULTIPLIER COMPONENTSBACKGROUND This invention relates to an improved gating apparatus for anelectron multiplier of a mass spectrometer.

The mass spectrometer is an instrument that permits rapid analysis ofmolecular species by measurement of the masses of the different ionsafter ionization of the molecules. In operation, a small amount of gasto be analyzed is admitted through a sample inlet into an ionizationchamber where the gas is ionized by electrons emitted from a filament.The ions are then directed or accelerated by an electric field from theionization chamber and into a region where the ions are separatedaccording to their mass to charge ratio (rule). The ions then impingeupon the cathode of an electron multiplier to achieve a gain of orgreater at the output. The output signal is then synchronized on anoscilloscope or gated to an analog for strip chart recording to indicatethe mass spectrum of the gas under analysis. A multiplier having morethan one gate gives the spectrometer a built-in capability to monitormultiple mass peaks of the spectrum simultaneously by the addition ofanalog scanners. Each scanner is capable of scanning the mass range from0 to 750 atomic mass units, or any portion thereof. A Bendix Time ofFlight Mass Spectrometer, Model 3012, is one mass spectrometer whichallows up to six analog scanner plug-in units to be used simultaneouslywith the oscilloscope output.

The ability to distinguish between mass peaks on the spectrum is calledresolution and, the better the resolution, the better the ability toidentify the constituents of the sample under analysis. Because ofimprovements to other portions of the mass spectrometer to improveresolution, the precise arrangement of the input and output componentsof the electron multiplier has become important. These components, e.g.the cathode at the input and the gating plates at the output when notprecisely arranged, adversely affect the resolution between massspectrum lines. For example, when the output gates are not preciselyarranged, portions of the output signals are misdirected and/or lost,resulting in peak broadening of the mass spectrum (i.e., the massspectrum line does not return to the base reference line between masspeaks). Further, the arrangement of the cathode at the multiplier inputmust be precise to maintain the proper separation of incoming signals.The efiects and advantages of precisely arranging the cathode isdisclosed in copending application entitled Mass Spectrometer HavingMeans Compensating for Electron Transit Time Across the Cathode of theElectron Multiplier" by D. C. Damoth and W. H. Shriner, filed Sept. l5,I969, Ser. No. 858,058.

The arrangement of the cathode and gating plates with precision is aproblem because tolerances such as, 10.00l inch and angles less than I",are difficult to obtain. Also, it is different to obtain preciseuniformity of arrangement from one multiplier to another. Therefore, thecomponents to be combined with each multiplier are individually adaptedto each other when each spectrometer is assembled. This results in atime-consuming, expensive and inefiicient method of assembly. Further,the present mounting methods and apparatus are not precise enough toobtain the improved resolution between mass peaks required by theadvancing biological sciences.

SUMMARY OF THE INVENTION To improve the performance of a massspectrometer, the resolution between mass peaks of the spectrum at thespectrometer output is improved by precisely arranging (spacing) thegating components of the electron multiplier so that electrons whengated to a particular collecting anode strike that anode. The inventionis characterized by an assembly which includes mounting plates havingrigidly fixed and precisely located arms or supports extending from themounting plates so that gating components, such as the gating wall,gating electrode, anode plate, and collecting plate secured to thesupports, are positioned in their proper places. The advantage of theassembly is that the precise alignment and spacing of components iseasily accomplished in one operation by precisely locating the holes forthe arms by jig boring the mounting plates. Jig boring also facilitatesduplication of the precise location of the holes from plate to plate.The invention is especially useful in improving the performance of thegating apparatus shown in US. Pat. No. 3,049,638 and for preciselyorienting the cathode as required in the aforementioned copendingapplication.

Accordingly, it is an object of this invention to provide an apparatuswhich permits precisely orienting the gating components associated withan electron multiplier.

It is another object of this invention to provide an apparatus whichpermits precisely orienting the cathode of an electron multiplier.

It is still another object of this invention to improve the performanceof a gating apparatus for charged particles.

It is a further object of this invention to improve the performance of amass spectrometer.

It is still a further object of this invention to improve the resolutionbetween mass peaks over the entire mass spectrum range.

The above and other objects and features of the invention will becomeapparent from the following detailed description taken in conjunctionwith the accompanying drawings and claims which form a part of thespecification.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagrammatic view of a timeof flight mass spectrometer utilizing the invention.

FIG. 2 is a more detailed partial plan view of the gating apparatusshown in FIG. 1.

FIG. 3 is a top view of the matrix assembly which precisely locates thegating components of a gating apparatus with respect to the dynode of anelectron multiplier.

FIG. 4 is a side view of the matrix assembly showing the preciselylocated jig boreholes which hold the anns that support the gatingcomponents.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to the drawings, FIG.I shows a mass spectrometer of the type described in US. Pat. Nos.2,765,408 and 2,685,035. Molecular species entering the ionizing regionI from the sample inlet 3 are ionized by electrons from filament 5. Theions 10 are then accelerated into the drift tube 7 by accelerating grids9. Because of the length of the drift tube 7 and the differentvelocities of the ions, the ions are separated according to their massto charge ratio (m/e) striking the cathode 20 at different times. Thedifferent times of flight (T) for ions of different masses (m) to travelfrom the ionizing region to the cathode may be calculated mathematicallyfrom the equation T=k(m/e)" where k is a constant depending on physicaldimensions. For example, the time of flight of a singly charged nitrogenion m=28 atomic mass units in a Bendix Time of Flight Mass Spectrometeris 5 microseconds and under usual conditions the time width of thenitrogen pulse at the spectrometers output is about 0.015 microsecond(about 0.060 microsecond for atomic species of 400 AMU). A magneticelectron multiplier 30 (e.g. U.S. Pat. Nos. 3,049,638 and 2,841,741) isused to detect and amplify the ion bunches. The ions pass throughaperture 2 of grid 4 and strike the cathode 20 to produce secondaryelectrons 40. The cathode 20 may be disposed at an acute angle with aplane that is perpendicular to the longitudinal axis 8 of the drift tube7 to decrease the time interval for all the electrons, propogated fromthe cathode by ions of the same mass, to strike the dynode. Theelectrons emitted from the cathode then follow a cycloidal path 41 underthe influence of the mutually perpendicular electric and magnetic fieldsin the multiplier to strike the dynode strip 31 multiplying in number toachieve a gain of approximately I0 K The resulting output signal is thensynchronized on an oscilloscope, or gated (gates 35) to an analog (notshown) for strip chart recording. Plates 70 and arms 80 form a matrixthat precisely locates, with respect to each other, the gates 35, (andgating components), the field strip 32, dynode 31 and rail 50. Thematrix may also be used at the input to the electron multiplier 30 toprecisely angle the cathode 20 where the electron multiplier is of thetype disclosed in the aforementioned copending application. At theinput, the arms 80 are placed in the holes 72 having predeterminedlocations so that when the arms 80 are placed in the holes 72, thecathode is held in a position'relative to the location of the holes 72.

FIG. 2 shows a gating assembly 35, of the type shown in U.S. Pat. No.3,049,638, which receives electrons from the electron multiplier 30.Mounting plate 70, which forms part of the matrix assembly thatprecisely arranges the gating components, is shown in phantom lines.Only a portion of the field strip 32 and dynode 31 of the multiplier 30are shown. The field strip 32 has an intermediate electrode 34 whichreceives a potential from a power source 61 and has an electrode 36 atone end which is grounded. The dynode 31 has an intermediate electrode42 and an electrode 44 at one end of the dynode which receive potentialsfrom a power supply 33.

The gating assembly 35 includes the rail 50 which is aligned with dynode31 and connected to electrode 44. The magnetic field of the electronmultiplier 30 extends through the rail 50 so that the cycloidal path ofthe electrons continues, but is changed a predetermined amount by anelectric field between the rail 50 and gates, so that electrons do notstrike the rail 50. Precisely spaced from the rail 50 and apart fromeach other are gating walls 52 which define the gating area. The gatingwalls 52 are connected to ground and electrode 36 of the field strip 32.Between the gating walls 52 are a plurality of gating electrodes 54 eachof which is precisely spaced from rail 50 and each of which has atransverse rod 56 secured (preferably welded) to one end thereof.Connected to each of the gating electrodes 54 is a pulser 58 whichmaintains the electrodes 54 at zero potential until it is desired todivert the path of the electron cycloid 41 to a collecting anode 62 byapplying a negative potential (preferably a negative voltage greaterthan that applied to the rail 50, such as -70 volts) to the appropriategating electrode 54. Precisely spaced from each electrode 54 is an anodeplate 60 which is maintained at a constant voltage (preferably anegative voltage greater than that applied to the collecting anode, suchas l volts) by power supply 61. Adjacent to each anode plate 60 in thecollecting anode 62 which is connected to an output device, (not shown)such as an oscilloscope, recorder or other indicating and/or actuatingdevices. The anode plates 60 are preferably spaced from collectinganodes 62 a precise distance, which is less than the cycloid height toprovide sufficient space for the electrons traveling along cycloidalpath 41 (diverted by a negative pulse to the proper electrode 54) tostrike the collecting anode 62. The gating electrodes 54 are preciselyspaced from the anode plates 60. collecting anodes 62 and ground plates64 to establish the proper electric field therebetween. A grounded plate64 is precisely spaced from the anode plate 60 and collecting anode 62to prevent electrons directed into one output to cross over into anotheroutput. The power supplies 33, 61 and 58, supply the voltages to thegating components to establish at predetermined time intervals theelectric fields necessary to direct electrons to the proper output.Although 3 outputs are shown, more or less may be used as desired.

FIG. 3 is a plan view of a portion of the matrix assembly whichprecisely locates the gating components. Mounting plates 70 have aplurality of holes 72, at predetermined locations. Mounted in the holes72 are arms 80. The arms are preferably wires having a diameter lessthan 0.060 inches and in their preferred positions extend perpendicularfrom the surface of the plate 70 and in parallel relationship to eachother. Secured to arms 80 (preferably welded) are the field strip 32,gating wall 52, collecting anode 62, anode plate 60, and ground plate64. Not shown is the dynode 31 which is also secured to an arm 80. Theholes 72 are precisely oriented so that when the arms, holding thegating components, are placed in the holes 72 and secured in place tothe mounting plates 70, the gating components are arranged in theirproper relationship with respect to field strip, dynode and each other.

FIG. 4 is a side view of the matrix assembly showing the location of theholes 72 in mounting plate 70. By locating the arms in the preciselyarranged holes 72, the gating components held by the amis are alsoprecisely arranged. The gating components shown are the gating wall 52,collecting anode 62, anode plate 60, ground plate 64 and rail 50.

The preferred method of making and locating the holes 72 is by jigboring. Jig boring the holes 72in each plate 70 precisely locates theholes in each plate without repeated time-consuming measurement.Further, accurate duplication from plate to plate which facilitatesalignment of the arms 80 is achieved.

Although one mounting plate 70 could be used, two mounting plates arepreferred, with supports extending between them. In this manner, a moreprecisely arranged matrix assembly is formed. I

OPERATION In describing how the matrix assembly improves the performanceof the gating apparatus 35, reference is made to FIG. 2 and thecycloidal path 41 of the electrons as they leave the dynode 31. Thecycloid is formed through the cooperation of the electrical and magneticfield existing between the field strip 32 and the dynode 31. At eachcontact point on the cycloid with the dynode 31 secondary electrons areemitted resulting in a multiplying action. The cycloid moves in adirection toward electrode 44. The magnitude of the cycloid may bevaried by varying the voltage applied to the electrode 34 of the fieldstrip 32. The cycloidal path of the electrons is lifted from the dynodestrip 31 by the electric field established by electrodes 42, 44, 36 and34. By the time the electrons reach the rail 50, they are traveling in acycloidal path 41 that has been elevated a predetermined amount by theapplication of a proper electric field. To maintain a uniform electricfield so that the electron cycloid travels parallel to rail 50, thegating electrode must be exactly spaced from the rail. This isaccomplished by arms 80 (FIG. 4) which hold the electrodes and rails.The electrons continue to travel in a cycloidal path parallel to therail 50 until one of the gating electrodes is pulsed in an appropriatemanner. For example, when gating electrode 54 of output gate number 3receives a negative voltage greater than the voltage applied to the rail50 from pulser 58, the cycloid is diverted towards gating anode 62. Anelectric field between the gating anode 62 and anode plate 60 thendirects the electrons towards the collecting anode 62. To preventelectrons from bypassing the collecting anode 62 on the side adjacent tothe gating electrode 54, a grounded plate 64 is precisely located byarms 80 (FIG. 3) between the collecting anode of gate 3 and the anodeplate 60 of gate 2. So that no electrons are misdirected or miss theirintended targets, it is essential that all the gating electrodes 54,anode plates 60, collecting anodes 62, and ground plates 64 be preciselyspaced from each other. The matrix assembly makes the precise spacingpossible.

While a preferred embodiment of the invention has been disclosed, itwill be apparent to those skilled in the art that changes may be made tothe invention as set forth in the appended claims, and, in some cases,certain features of the invention may be used to advantage withoutcorresponding use of other features. Accordingly, it is intended thatthe illustrative and descriptive materials herein be used to illustratethe principles of the invention and not to limit the scope thereof.

Having described the invention, what is claimed is:

1. In combination with a magnetic electron multiplier of the typewherein electrons traveling in cycloidal paths in the space between afield strip and a dynode are accelerated from the dynode to a gatingapparatus having a gating components a plurality of wires extendingbetween said plates in fixed positions. said wires in generally paralleland spaced relationship with respect to each other, each of said wireshaving attached thereto only one component of said gating components, sothat said gating wall, collecting anode, anode plate and ground plateare precisely spaced and oriented with respect to each other and saidfield strip and said dynode.

1. In combination with a magnetic electron multiplier of the typewherein electrons traveling in cycloidal paths in the space between afield strip and a dynode are accelerated from the dynode to a gatingapparatus having gating components comprising a gating wall, acollecting anode, an anode plate, a ground plate and means for mountingsaid gating components in predetermined relationship and wherein theelectrons are directed, by potentials applied to said gaitingcomponents, to impinge said collecting anode, the improvement whereinsaid mounting means comprises: a pair of plates disposed in spacedrelationship with respect to each other at one end of said dynode, saidplate adjacent to and generally perpendicular to said dynode; and aplurality of wires extending between said plates in fixed positions,said wires in generally parallel and spaced relationship with respect toeach other, each of said wires having attached thereto only onecomponent of said gating components, so that said gating wall,collecting anode, anode plate and ground plate are precisely spaced andoriented with respect to each other and said field strip and saiddynode.